Interleaver for IEEE 802.11n standard

ABSTRACT

A MIMO wireless system includes a transmitter having a parser that parses a bit stream into multiple spatial data streams and multiple interleavers corresponding to the multiple spatial data streams, where each interleaver interleaves the bits in the corresponding spatial data stream by performing frequency rotation after an interleaving operation, to increase diversity of the wireless system. The MIMO wireless system also includes a receiver that has deinterleavers that deinterleaves spatial bit streams transmitted by the transmitter.

FIELD OF THE INVENTION

The present invention relates generally to data communication, and moreparticularly, to data communication with transmission diversity usingOrthogonal Frequency Division Multiplexing (OFDM) in multiple antennachannels.

BACKGROUND OF THE INVENTION

In wireless communication systems, antenna diversity plays an importantrole in increasing the system link robustness. OFDM is used as amultiplexing technique for transmitting digital data using radiofrequency signals (RF). In OFDM, a radio signal is divided into multiplesub-signals that are transmitted simultaneously at different frequenciesto a receiver. Each sub-signal travels within its own unique frequencyrange (sub-channel), which is modulated by the data. OFDM distributesthe data over multiple channels, spaced apart at different frequencies.

OFDM modulation is typically performed using a transform such as FastFourier Transform (FFT) process wherein bits of data are encoded in thefrequency-domain onto sub-channels. As such, in the transmitter, anInverse FFT (IFFT) is performed on the set of frequency channels togenerate a time-domain OFDM symbol for transmission over a communicationchannel. The IFFT process converts the frequency-domain data for eachsub-channel into a block of time-domain samples which are later onconverted to an analogue modulating signal for an RF modulator. In thereceiver, the OFDM signals are processed by performing an FFT process oneach symbol to convert the time-domain data into frequency-domain data,and the data is then decoded by examining the phase and amplitude of thesub-channels. Therefore, at the receiver the reverse process of thetransmitter is implemented. Further, transmission antenna diversityschemes are used to improve the OFDM system reliability. Such transmitdiversity schemes in OFDM systems are encoded in the frequency-domain asdescribed.

OFDM has been selected as the basis for the high speed wireless localarea network (WLAN) standards by the IEEE 802.11a standardization group,and is also being considered as the basis for the high throughput WLAN802.11n. A typical transmitter for a conventional OFDM MIMO systemimplementing WLAN 802.11n comprises a channel encoder, a puncturer, aspatial parser, and multiple data stream processing paths. Each datastream processing path comprises an interleaver, a constellation mapper,an IFFT function, and guard interval insertion window and an RFmodulator.

For parser and interleaver portion of the system, coded and puncturedbits are interleaved across spatial streams and frequency tones. Thereare two steps to the space-frequency interleaving: spatial streamparsing and frequency interleaving. First, encoded and punctured bitsare parsed to multiple spatial streams by a round-robin parser. Theparser sends consecutive blocks of bits to different spatial streams ina round-robin fashion starting with the first spatial stream. Second,all encoded bits are interleaved by a separate block interleaver foreach spatial stream, with a block size corresponding to the number ofbits in a single OFDM symbol. The block interleavers are based on the802.11a interleaver, with certain modifications to allow for multiplespatial streams and 40 MHz transmissions.

The interleaver is defined by a two-step permutation. The firstpermutation ensures that adjacent coded bits are mapped onto nonadjacentsubcarriers. The second permutation ensures that coded bits are mappedalternately onto less and more significant bits of the constellation andthereby long runs of low reliability (LSB) bits are avoided. Adeinterleaver in a receiver performs the inverse operation, and is alsodefined by two permutations corresponding to the two interleaverpermutations.

Such conventional system provides write in block, one column rotationfor multiple antennas transmission, and PAM order rotation within acolumn as described in S. A. Mujtaba, “TGn Sync Proposal TechnicalSpecification,” a contribution to IEEE 802.11 11-04-889r1, November 2004and Manoneet Singh et al. and Bruce Edwards et al., “WWiSE proposal:High throughput extension to the 802.11 Standard,” a contribution toIEEE 802.11 11-04-0886r4, November 2004, (incorporated herein byreference). However, because the columns are rotated by only one column,adjacent bits only 3 and 6 sub-carriers apart for 20 MHz and 40 MHzsystems, respectively. As a result, in a correlated channel, thediversity gain is not fully utilized.

Another conventional transmitter design includes a channel encoder, apuncturer, a frequency interleaver, a spatial parser, and two datastream processing paths. Each data stream processing path comprises aconstellation mapper, an IFFT function, guard interval insertion windowand an RF modulator. The interleaver performs interleaving on twoconsecutive OFDM symbols before they are parsed onto two differentantennas. The relation for the first permutation is:i=N _(row)×(k mod N _(column))+floor(k/N _(column))

where N_(column)=32, N_(row)=2N_(CBPS)/N_(column)

After the interleaving, the spatial parser parses the interleaved bitsin group by a round robin fashion to different spatial streams. Thegroup size equals to the number of bits in one QAM symbol. For example,for 64 QAM, 6 bits will be parsed onto one spatial stream and the next 6bits will be parsed onto another spatial stream. However, such atransmitter is not flexible enough to accommodate different channelcoding and modulation schemes on different special streams.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides an interleaver designfor a transmitter that is flexible enough to accommodate differentchannel coding and modulation schemes on different special streams.

Accordingly, an MIMO wireless system according to an embodiment of thepresent invention includes a transmitter having a parser that parses abit stream into multiple spatial data streams and multiple interleaverscorresponding to the multiple spatial data streams, where eachinterleaver interleaves the bits in the corresponding spatial datastream by performing a frequency rotation, which is equivalent tomultiple column rotations and at least one row rotation, to increasediversity of the wireless system. The MIMO wireless system also includesa receiver that has deinterleavers that deinterleavers spatial bitstreams transmitted by the transmitter.

These and other features, aspects and advantages of the presentinvention will become understood with reference to the followingdescription, appended claims and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a functional block diagram of an embodiment of an OFDMMIMO transmitter having a transmitter data path for 2-antenna MIMO in a20 MHz channel, according to the present invention.

FIG. 1B shows a functional block diagram of an embodiment of an OFDMMIMO transmitter having a transmitter data path for 2-antenna MIMO in a40 MHz channel, according to the present invention.

FIG. 2 shows a flowchart of the steps of an embodiment of aninterleaving process in a MIMO transmitter according to the presentinvention.

FIG. 3 shows a functional block diagram of example interleaversaccording to another embodiment of the present invention.

FIG. 4 shows a functional block diagram of an embodiment of an OFDM MIMOreceiver including deinterleavers according to the present invention.

FIGS. 5A-B show example simulation results in 20 MHz channels.

FIG. 6 shows a functional block diagram of an embodiment of an OFDM MIMOtransmitter according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the present invention provides a method of bit streaminterleaving for a MIMO system. The interleaving method uses frequencyrotation to better explore the diversity of the wireless system andsimplify the implementation.

FIG. 1A shows an example block diagram of an OFDM MIMO system 100 (e.g.,20 MHz channel) implementing WLAN 802.11n, according to an embodiment ofthe present invention. The system 100 includes a transmitter 101 and areceiver 102. The transmitter 101 comprises a channel encoder 103, apuncturer 104, a spatial parser 106, and two data stream processingpaths 107. Each data stream processing path 107 comprises an interleaver(e.g., interleaver 108A for a first processing path, and interleaver108B for a second processing path), a constellation mapper 110, an IFFTfunction 112, and guard interval insertion window 114 and an RFmodulator 116. For the parser 106 and the interleavers 108A, 108Bportions of the transmitter, coded and punctured bits are interleavedacross spatial streams and frequency tones. There are two steps to thespace-frequency interleaving: spatial stream parsing and frequencyinterleaving.

Conventionally, encoded and punctured bits are parsed to multiplespatial streams by a round-robin parser wheres=max{N _(BPSC)/2,1}  (1)

such that s is the number of bit parsed onto one antenna each round, andN_(BPSC) is the number of coded bits per subcarrier. A conventionalparser sends consecutive blocks of s bits to different spatial streamsin a round-robin fashion starting with the first spatial stream. Allencoded bits are conventionally interleaved by a separate blockinterleaver for each spatial stream, with a block size corresponding tothe number of coded bits in a single OFDM symbol, N_(CBPS). Theconventional block interleavers are based on the 802.11a interleaver,with certain modifications to allow for multiple spatial streams and 40MHz transmissions.

The basic interleaver array has N_(row) rows and N_(column) columns andN_(BPSC) is the number of coded bits per subcarrier (e.g. N_(BPSC)=1 forBPSK, 2 for QPSK, 4 for 16 QAM, etc), wherein the interleaver parametersare shown in Table 1 below.

TABLE 1 Interleaver Parameters N_(column) N_(row) 20 MHz 16 3 N_(BPSC)channels 40 MHz 18 6 N_(BPSC) channels

A conventional interleaver is defined by a two-step permutation. Thefirst-step permutation (first permutation) ensures that adjacent codedbits are mapped onto nonadjacent subcarriers. The first permutation ismodified from the 802.11a interleaver such that the column indexes inthe array are rotated by one column for each spatial stream. Thesecond-step permutation (second permutation) ensures that coded bits aremapped alternately onto less and more significant bits of theconstellation and thereby long runs of low reliability (LSB) bits areavoided.

Relations (2) and (3) below define a conventional interleaver, whereinthe index of the coded bit before the first permutation is denoted by k,and i is the index after the first and before the second permutation. Inthe conventional interleaver, the first permutation is defined byrelation (2) below:i=N _(row)×(((k mod N _(column))+i _(SS))mod N _(column))+floor(k/N_(column)),  (2)k=0, 1, . . . , N _(CBPS)−1,

where i_(SS)=0, 1, . . . , N_(SS)−1, is the index of the spatial streamon which this interleaver is operating. The insertion of i_(SS) is amodification of the 802.11a interleaver. This results in a “columnoffset” in the de-interleaving process. That is, bits are read in byrows and out by columns, but starting with column i_(SS) in acolumn-cyclic fashion.

Further, conventionally the second permutation is defined by relation(3) below, wherein j is the index after the second permutation, justprior to modulation mapping:j=s×floor(i/s)+(i+N _(CBPS)−floor(N _(column) ×i/N _(CBPS)))mod s,  (3)i=0, 1, . . . , N _(CBPS)−1,

where s is determined according to relation (4) below:s=max(N _(BPSC)/2,1).  (4)

Similarly, a deinterleaver in a receiver performs the inverse relation,and is defined by a first-step permutation and a second-step permutationcorresponding to the conventional interleaver permutations above.Relations (5) and (6) define these first and second permutations for aconventional deinterleaver, wherein the index of the original receivedbit before the first permutation is denoted by j, and i is the indexafter the first and before the second permutation.

Conventionally, the first permutation in the deinterleaver is defined byrelation (5) below:i=s×floor(j/s)+(j+floor(N _(column) ×j/N _(CBPS)))mod s,  (5)j=0, 1, . . . , N _(CBPS)−1,

where s is as defined in relation (4) above. The first permutation inrelation (5) is the inverse of the permutation in relation (3) above.

Conventionally, the second permutation in the deinterleaver is definedby relation (6) below, where k is the index after the secondpermutation:k=N _(column)(i mod N _(row))+(floor(i/N _(row))−i _(ss) +N_(column))mod N _(column),  (6)i=0, 1, . . . , N _(CBPS)−1.

The second permutation in relation (6) is the inverse of the interleaverpermutation in relation (2) above.

As noted, the conventional system provides write in block, columnrotation for multiple antennas transmission, and PAM order rotationwithin a column. However, because the columns are rotated by only onecolumn, adjacent bits only e.g. 3 and 6 sub-carriers apart for 20 MHzand 40 MHz systems. As a result, in a correlated channel, the diversitygain is not fully utilized.

In the commonly assigned patent application Ser. No. 11/104,808, filedon Apr. 12, 2005, an improved interleaving process is described whichincreases the column rotation to the largest possible distance within ablock to fully explore the diversity of the wireless system. In suchimproved interleaving process, in a first permutation, the columnrotation is changed from one column rotation to((N_(column)/N_(ss))×i_(ss)) column rotations, where N_(ss) is the totalnumber of spatial data streams and i_(ss) is index of spatial datastream which ranges from e.g. 0 to N_(ss)−1. As such, in contrast to theconventional interleaving relation (2) above, the first permutation inthe improved interleaving process is defined by relation (7) below:i=N _(row)×(((k mod N _(column))+floor(N _(column) /N _(ss))×i _(ss))modN _(column))+floor(k/N _(column))  (7)where k=0, 1, . . . , N_(CBPS)−1.

On the receiver side, a deinterleaving process performs the reverseoperation for de-interleaving the received bits. In contrast to theconventional deinterleaving relation (6) above, the second permutationin the deinterleaver is defined by relation (8) below:k=N _(column)×(i mod N _(row))+(floor(i/N _(row))−floor(N _(column) /N_(ss))×i _(ss) +N _(column))mod N _(column)  (8)wherein i=0, 1, . . . , N_(CBPS)−1.

For example, if two data streams are to be transmitted, using theimproved interleaving process. The adjacent data bits are separated 8columns apart for different data streams in a 20 MHz channel. In anotherexample, the adjacent data bits are separated 9 columns apart fordifferent data streams in a 40 MHz channel.

In one example transmitter where there are multiple spatial streams, ablock of bits in first data stream is transmitted without any rotationin that block. Conventionally, each remaining spatial stream istransmitted after i_(ss) column rotation relative to the first spatialstream. However using the improved interleaving process, each remainingspatial stream is transmitted after multiple column rotations, whereinthe number of rotations is the number of columns in the interleaverarray divided by the number of spatial streams.

In another example transmitter where there are two spatial streams, andone spatial stream is transmitted over a first antenna, and the otherover a second antenna, a first block of bits is transmitted over thefirst antenna without any rotation in that block. Conventionally, forthe second antenna, data is transmitted with one column rotation wherethe second column is rotated to the first column, and so one, so thatall the columns are shifted/rotated left by one column. Using theimproved, however, for the second antenna, the number of rotations isthe number of columns in the interleaver array divided by the number ofantennas. For example, for a 20 MHz transmitter having two antennas, theinterleaver array comprises 16 columns and three rows of bits. Using theimproved interleavers, for the second antenna the number of rotations isthe number of columns (16) divided by the number of antennas (2),resulting in 8 column rotations. As such, columns 9 through 16 areshifted into the first part (first 8 columns) of the array block, andcolumns 1 through 8 are shifted into the second part (second 8 columns)of the array block for transmission.

The MIMO system performance (e.g., packet error rate vs. signal to noiseratio) using multiple column rotation interleaving in the improvedinterleavers is improved in comparison to a conventional system with onecolumn rotation interleaving. This is because in OFDM differentsubcarriers are used and when bits are rotated by multiple columns,adjacent bits are separated further in the spatial domain and in thesubcarrier space, reducing fading in transmission channels.

Using multiple column rotation according to the improved interleavingprocess, two adjacent bits have less probability of seeing the samechannel. As such, in the receiver when the received data bits arede-interleaved for convolution decoding, if one received bit has lowenergy (bad bit) because of transmission in a fading channel, and anadjacent bit has high energy (good bit), the good bit can be used torecover the bad bit by convolution decoding.

With the conventional one column rotation interleaving, the adjacentdata bits are spatially close and can face the same bad transmissionchannel. In a case where there are several continuous bits that face thesame bad channel, it is difficult for the receiver to recover the bitsby convolution decoding. However, with multiple column rotation in theimproved interleaving process, adjacent bits are spatially separatedsuch that they are less likely to be transmitted in the same bad/fadingchannel. As such, if a bit is transmitted in a bad channel, and theadjacent bits are transmitted via good channels, decoding in thereceiver can still recover the bit transmitted via the bad channel usingthe bits transmitted in the good channels.

In the commonly assigned patent application Ser. No. 11/292,851entitled: “An improved interleaver design for IEEE 802.11n standard”,(incorporated herein by reference), to further separate adjacent bitsinto different sub-band and different spatial stream, in addition tolarger column rotation an additional row rotation is performed ondifferent spatial streams, which result in the change of relation (2)above to relation (10) below:i=N _(row)*(((k mod N _(column))+floor(N _(column) /N _(ss))*i _(ss))modN _(column))+(floor(k/N _(column))+ceil(N/N _(ss) *i _(ss))*N_(BPSC))mod N _(row)  (10)

where k=0, 1, . . . N_(CBPS)−1, and

-   -   i_(ss)=0 . . . N_(ss)−1 where N_(ss) is the number of spatial        data streams.

Accordingly, the corresponding deinterleaving relation is modified intorelation (11) below, as:k=(N _(column)*(i mod N _(row))+(floor(i/N _(row)−floor(N _(column) /N_(ss))*i _(ss))mod N _(col) +N _(column)*(N−ceil(N/N _(ss) *i _(ss)))*N_(BPSC))mod N _(CBPS),  (11)

wherein i=0, 1, . . . N_(CBPS)−1

-   -   i_(ss)=0 . . . N_(ss)−1, where N_(ss) is the number of spatial        data streams.        Improved Interleaver/Deinterleaver

According to the present invention, an embodiment of a further improvedinterleaving process is implemented in the system 100 of FIG. 1A, tofurther separate adjacent bits into different sub-band and differentspatial stream, wherein in addition to larger column rotations (e.g.,Ncol/Nss), one or more additional row rotations on different spatialstreams with unification of row and column rotation by a frequencyrotation, are performed. Row cyclic rotation is the same operation as incolumn cyclic rotation except the operation is on the rows instead ofcolumns.

Instead of viewing the problem in the interleaver block (array), theimplementation of the column and row interleaving according to anembodiment of the present invention includes two steps: First,performing the same interleaving for all the data streams usingIEEE802.11a interleaver parameters. The output data bits are then mappedto different sub-carriers of an OFDM symbol. Second, different cyclicshift are performed along the frequency sub-carriers for different datastreams to obtain the same effect of column and row rotation in thefirst permutation of the interleaver operation.

In one example of the further improved interleaving process according toan embodiment of the present invention, such is accomplished byrelations (12) through (14) below for the interleavers 108A, 108B.

The further improved interleaving process for the interleavers 108A,108B, is implemented in three steps according to relations (12) through(14):i=N _(row)×(k mod N _(column))+floor(k/N _(column)), k=0, 1, . . . , N_(CBPS)−1,  (12)j=s×floor(i/s)+(i+N _(CBPS)−floor(N _(column) ×i/N _(CBPS)))mod s, i=0,1, . . . , N _(CBPS)−1,  (13)

where s is determined according to s=max(N_(BPSC)/2,1),r=(j−((2×i _(ss))mod 3+3×floor(i _(ss)/3))×N _(rot) ×N _(BPSC))mod N_(CBPS) , j=0, 1, . . . , N _(CBPS)−1,  (14)

wherein in relations (12) to (14), the index of the coded bit before thefirst permutation is denoted by k, and i is the index after the firstand before the second permutation, and j is the index after the secondpermutation and before the third permutation, and r is the index afterthe third permutation, i_(SS)=0, 1, . . . , N_(SS)−1 is the index of thespatial stream on which this interleaver is operating and N_(rot) is abase rotation number in use. We choose N_(rot) equal to 11 and 29 for 20MHz and 40 MHz operation respectively. The i_(ss) parameter controls therotation of each stream such that for the first stream, there will be nofrequency rotation

Relation (12) above ensures that adjacent coded bits are mapped ontononadjacent sub-carriers. Relation (13) above ensures that coded bitsare mapped alternately onto less and more significant bits of theconstellation whereby long runs of low reliability (LSB) bits areavoided. Relation (14) represents the frequency rotation operation.

Accordingly, the corresponding deinterleaving relations for thedeinterleavers 118A, 118B in the example receiver 102, are according torelations (15) to (17) below:x _(d)=(x _(dd)+((2×i _(ss))mod 3+3×floor(i _(ss)/3))×N _(rot) ×N_(BPSC))mod N _(CBPS)x _(dd)=0, 1, . . . , N _(CBPS)−1,  (15)j _(d) =s×floor(j _(dd) /s)+(j+floor(N _(column) ×j _(dd) /N_(CBPS)))mod s j _(dd)=0, 1, . . . , N _(CBPS)−1,  (16)i _(d) =N _(column) ×i _(dd)−(N _(CBPS)−1)×floor(N _(column) ×i _(dd) /N_(CBPS)), i _(dd)=0, 1, . . . , N _(CBPS)−1,  (17)

wherein i_(dd), j_(dd) represent the index before operation of each stepaccordingly, the index of the coded bit before the first permutation isdenoted by x_(dd), before the second permutation and after the firstpermutation by j_(dd) and before third permutation and after secondpermutation by i_(dd), and x_(d) is the index after the first and beforethe second permutation, and j_(d) is the index after the secondpermutation and before the third permutation, and i_(d) is the indexafter the third permutation.

Relations (15) through (17) above perform inverse steps of relations(14) through (12), respectively.

For example, in a 20 MHz channelization case, the block size of theinterleaver is 3*N_(BPSC) rows and 16 columns. For a two data streamcase, the interleaver for the first data stream can be the same as anIEEE802.11a interleaver. However, for the second data stream, inaddition to the usual 802.11a interleaver operation, according to anembodiment of the present invention an additional step of frequencyrotation is added before the IFFT operation.

A simple example of BPSK case where N_(BPSC)=1 is now described. Thefollowing example Tables 2(a)-(c) show the bit positions for a BPSKmodulated OFDM symbol before and after the interleaving operation.

TABLE 2(a) bit position in the interleaver block 1 2 3 4 5 6 7 8 9 10 1112 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 3536 37 38 39 40 41 42 43 44 45 46 47 48

The bits are written in rows and read out in columns, such that afterthe block interleaving, the bit positions on the subcarriers are shownin Table 2(b) below:

TABLE 2(b) 1 17 33 2 18 34 3 19 35 4 20 36 5 21 37 6 22 38 7 23 39 8 2440 9 25 41 10 26 42 11 27 43 12 28 44 13 29 45 14 30 46 15 31 47 16 3248

The bit positions after the frequency rotation is shown in Table 2(c)below:

TABLE 2(c) 9 25 41 10 26 42 11 27 43 12 28 44 13 29 45 14 30 46 15 31 4716 32 48 1 17 33 2 18 34 3 19 35 4 20 36 5 21 37 6 22 38 7 23 39 8 24 40

FIG. 1B shows a functional block diagram of an embodiment of an OFDMMIMO transmitter 120 having a transmitter data path for 2-antenna MIMOin a 40 MHz channel, according to the present invention. The transmitter120 comprises a channel encoder 123, a puncturer 124, a spatial parser126, and two data stream processing paths 127. Each data streamprocessing path 127 comprises an interleaver (e.g., interleaver 128A fora first processing path, and interleaver 128B for a second processingpath), a constellation mapper 130, an IFFT function 132, and guardinterval insertion window 134 and an RF modulator 136. For the parser126 and the interleavers 128A, 128B portions of the transmitter, codedand punctured bits are interleaved across spatial streams and frequencytones. The interleavers 128A and 128B implement an embodiment of saidfurther improved interleaving process according to the present inventionas described herein.

FIG. 2 shows a flowchart of the steps of example operation of thetransmitter 101 of FIG. 1A (or transmitter of FIG. 1B) according to anembodiment of said further improved interleaving process provided by thepresent invention. According to the example in FIG. 2, the transmitter101 of FIG. 1A operates according to the steps of: source bit stream isreceived (step 200); the channel encoder 103 encodes data usingconvolutional encoding (CC) (step 202); the puncturer 104 punctures thebits from the CC to change the coding rate (step 204); the spatialparser 106 separates the data stream into several spatial streams (step206); then in each processing path 107 beyond the first path, aninterleaver 108B interleaves the bits using identical 802.11a alikeinterleaver plus an additional different frequency rotations accordingto an embodiment of the present invention (i.e., for different spatialstreams different rotations are used e.g. interleave the data across 48data subcarriers for 20 MHz channel, interleave the data across 108 datasubcarriers for 40 MHz channel, etc.) (step 208); the constellationmapper 110 groups/maps the interleaved bits into symbols using a GrayMapping Rule (e.g., BPSK groups 1 bit into one symbol; 64 QAM groups 6bits into one symbol, etc.) (step 210); the symbols are distributed ondata subcarrier of one OFDM symbol by an IFFT operation wherein the datasymbols are mapped onto each subcarrier for IFFT (step 212); the IFFTfunction 112 converts the frequency domain data to time domaintransmission data (step 214); the guard window 114 adds guard intervalto each OFDM symbol in time domain, to prevent inter symbol interference(step 216); and in the RF modulator 116 the signal is RF modulated andtransmitted through the channel via antennas 117 (step 218).

FIG. 3 shows a more detailed block diagram of a transmitter 300utilizing an interleaving process for multiple spatial stream paths,according to another embodiment of the present invention. Thetransmitter 300 includes a spatial parser 302, and multiple (i.e., 2 ton, e.g. n=4) spatial stream processing paths 304. Each path 304 includesan interleaver 306 and a bit to symbol mapper 308. Each interleavercomprises a block first permutation interleaver 310 and a secondpermutation interleaver 312. Then, for different spatial streams beyondthe first spatial stream processing path, an N-frequency rotation isperformed by the frequency rotation block 311.

After spatial parsing of the input bit stream into multiple spatialstreams by the parser 302, each spatial stream is processed in acorresponding spatial stream processing path 304. Then, the bitsprocessed in each spatial stream path 304 are transmitted via a channel(e.g., as in system 100 of FIG. 1A).

As noted, in the example of FIG. 3, the interleaver 306 in each spatialstream path beyond the first spatial stream processing path comprises ablock first permutation interleaver 310, a second permutationinterleaver 312 and a third permutation of frequency rotation 311,wherein the frequency rotation interleavers 311 are configured accordingto the present invention (e.g., relations 12 through 14 above).

Referring to FIG. 4, an example receiver 400 performs the reverseoperation, wherein the receiver 400 includes a de-interleaver 402 foreach spatial stream for de-interleaving the received bits of eachspatial stream according to the present invention. Each deinterleaver402 beyond the first path comprises a third permutation frequencyde-rotation 401, a second permutation deinterleaver 404 implementingrelation (5) above and first permutation deinterleaver 406A, 406B (e.g.,implementing relations 15 through 17 above).

Example simulations have verified the performance gains of the furtherimproved interleaving process according to the present invention in 20MHz MIMO channelizations. The coding and modulation modes for theexample simulations are listed in Table 3.

TABLE 3 Modulation and Coding Scheme (MCS) definition in simulationNumber of spatial streams Modulation Coding rate 2 64-QAM 5/6 2 64-QAM7/8

FIGS. 5A-B show example simulation comparative results for IEEE 802.11nchannel models BLOS, DLOS, ELOS and DNLOS for above listed modulationand coding combination. For sake of simplicity of example, perfectsynchronization, no RF impairment, and perfect channel estimation isassumed. Further, an MMSE detector is utilized for data streamseparation.

In the example shown in FIG. 5A, graphs 501, 502 and 503 represent CC5/6BNLOS performance according to said commonly assigned patent applicationSer. No. 11/292,851, CC5/6 BNLOS performance according to the presentinvention, and CC5/6 BNLOS performance according the prior art,respectively.

Similarly, graphs 504, 505 and 506 represent CC5/6 DNLOS performanceaccording to said commonly assigned patent application Ser. No.11/292,851, CC5/6 DNLOS performance according to the present invention,and CC5/6 DNLOS performance according the prior art, respectively.

Similarly, graphs 504, 505 and 506 represent CC5/6 ENLOS performanceaccording to said commonly assigned patent application Ser. No.11/292,851, CC5/6 ENLOS performance according to the present invention,and CC5/6 ENLOS performance according the prior art, respectively.

In the example shown in FIG. 5B, graphs 601, 602 and 603 represent CC7/8BNLOS performance according to said commonly assigned patent applicationSer. No. 11/292,851, CC7/8 BNLOS performance according to the presentinvention, and CC7/8 BNLOS performance according the prior art,respectively.

Similarly, graphs 604, 605 and 606 represent CC7/8 DNLOS performanceaccording to said commonly assigned patent application Ser. No.11/292,851, CC7/8 DNLOS performance according to the present invention,and CC7/8 DNLOS performance according the prior art, respectively.

Similarly, graphs 607, 608 and 609 represent CC7/8 ENLOS performanceaccording to said commonly assigned patent application Ser. No.11/292,851, CC7/8 ENLOS performance according to the present invention,and CC7/8 ENLOS performance according the prior art, respectively.

Various simulation results show that the simplified method has thesimilar performance as the scheme in commonly assigned patentapplication Ser. No. 11/292,851. The main performance improvementfactors are the frequency rotation among spatial data streams.

An improved interleaving process according to the present invention canbe implemented in a modified OFDM MIMO transmitter architecture 600 fora 20 MHz channel is shown in FIG. 6, according to another embodiment ofthe present invention. Compared to FIG. 1A, in the modified transmitterarchitecture of FIG. 6, the puncturing processing is performed after theparsing processing. In this case, two puncturers are utilized, onepuncturer per data stream processing path. With this modified structure,a MIMO system can transmit data streams with different coding ratessimultaneously.

The transmitter 600 includes a channel encoder 103, a spatial parser106, and two data stream processing paths 602. Each data streamprocessing path 602 includes a puncturer 104, an interleaver (e.g.,interleavers 108A and 108B), a constellation mapper 110, an IFFTfunction 112, and guard-interval insertion window 114 and an RFmodulator 116. For the parser 106 and the interleaver 108A/108B portionsof the transmitter, coded and punctured bits are interleaved acrossspatial streams and frequency tones.

As shown, in the transmitter 600, each spatial data stream path includesa puncturer 104, allowing different transmission rates for the twospatial streams (based on the channel conditions). In one example, onepuncturer 104 provides a convolutional code 1/2 for a first data stream,and the other puncture 104 provides a convolution code 3/4 for thesecond data stream. Using multiple puncturers 104 provides moreflexibility. For example, where there are two transmitter antennas, ifthe first antenna provides a better channel than the second antenna,then on the first antenna a high transmission data rate can be achieved,and on the second antenna somewhat lower data transmission rate isachieved. This combination makes a MIMO OFDM system according to thepresent invention more flexible and essentially allows optimization ofthe transmission.

As those skilled in the art will recognize, the example transmitter inFIG. 1B can be similarly modified such that the puncturing processing isperformed after the parsing processing, wherein two puncturers areutilized (one puncturer per data stream processing path), much the sameway as shown in FIG. 6 and described above.

As those skilled in the art will recognize, other implementations of thepresent invention are possible, and the present invention is not limitedto the example number of frequency rotations described above. Theselected set of parameters is chosen based on the specific number ofsubcarriers in the 802.11n system. In other systems with differentnumber of subcarriers and parsers, the principles of the presentinvention can be used while the specific rotation parameters can bedifferent.

The present invention has been described in considerable detail withreference to certain preferred versions thereof; however, other versionsare possible. Therefore, the spirit and scope of the appended claimsshould not be limited to the description of the preferred versionscontained herein.

1. A method of data communication in a wireless system, comprising the steps of: parsing a bit stream into multiple spatial data streams; interleaving the bits in each of one or more spatial data streams by performing frequency rotation after an interleaving operation, to increase diversity of the wireless system, wherein frequency rotation is performed according to relation: r=(j−((2×i _(ss))mod 3+3×floor(i _(ss)/3))×N _(rot) ×N _(BPSC))mod N _(CBPS) , j=0, 1, . . . , N _(CBPS)−1, wherein N_(BPSC) is the number of coded bits per subcarrier, N_(CBPS) is a block size corresponding to a number of coded bits in a single Orthogonal Frequency Division Multiplexing (OFDM) symbol, j is an index after a second permutation and before a third permutation, r is an index after the third permutation, i_(SS)=0, 1, . . . , N_(SS)−1 is the index of the spatial stream on which interleaving is operating, N_(ss) is the number of spatial streams, and N_(rot) is a base rotation number in use and transmitting the bits of each spatial data stream.
 2. The method of claim 1 wherein the number of frequency rotations for a spatial data stream is a function of the number of the total spatial data streams.
 3. The method of claim 1 wherein the steps of interleaving the bits in a spatial data stream includes the steps of performing a first interleaving permutation to ensure that adjacent coded bits are mapped onto nonadjacent subcarriers in one data stream for transmission, and a second permutation to ensure that coded bits are mapped alternately onto less and more significant bits of the constellation, and a third interleaving permutation providing the frequency rotation, varying on different spatial data streams to increase diversity of the wireless system.
 4. The method of claim 1, wherein the steps of interleaving the bits in a spatial data stream includes the steps of: (a) ensuring that adjacent coded bits are mapped onto nonadjacent sub-carriers, and (b) ensuring that coded bits are mapped alternately onto less and more significant bits of the constellation whereby long runs of low reliability (LSB) bits are avoided.
 5. The method of claim 4 wherein each spatial data stream interleaver array includes N_(row) rows and N_(column) columns of bits, wherein: the steps (a) of ensuring that adjacent coded bits are mapped onto nonadjacent sub-carriers is according to relation: i=N _(row)×(k mod N _(column))+floor(k/N _(column)), k=0, 1, . . . , N _(CBPS)−1, the steps (b) of ensuring that coded bits are mapped alternately onto less and more significant bits of the constellation whereby long runs of low reliability (LSB) bits are avoided is according to relation: j=s×floor(i/s)+(i+N _(CBPS)−floor(N _(column) ×i/N _(CBPS)))mod s, i=0, 1, . . . , N _(CBPS)−1, where s is determined according to s=max(N_(BPSC)/2,1), and k denotes the index of the coded bit before a first permutation.
 6. The method of claim 4 further including the steps of receiving the transmitted bits of each spatial bit stream, and deinterleaving the received bits according to relations: x _(d)=(x _(dd)+((2×i _(ss))mod 3+3×floor(i _(ss)/3))×N _(rot) ×N _(BPSC))mod N _(CBPS) , x _(dd)=0, 1, . . . , N _(CBPS)−1 j _(d) =s×floor(j _(dd) /s)+(j+floor(N _(column) ×j _(dd) /N _(CBPS)))mod s j _(dd)=0, 1, . . . , N _(CBPS)−1, i _(d) =N _(column) ×i _(dd)−(N _(CBPS)−1)×floor(N _(column) ×i _(dd) /N _(CBPS)), i _(dd)=0, 1, . . . , N _(CBPS)−1, wherein the index of the coded bit before the first permutation is denoted by x_(dd), and x_(d) is the index after the first and before the second permutation, and j_(d) is the index after the second permutation and before the third permutation, i_(d) is the index after the third permutation, and N_(CBPS) denotes a block size corresponding to the number of coded bits in a single OFDM symbol, i_(SS)=0, 1, . . . , N_(SS)−1 is the index of the spatial stream on which this interleaving is operating, and N_(rot) is a base rotation number in use.
 7. The method of claim 1 wherein the wireless system comprises a multiple-input multiple-output (MIMO) system.
 8. The method of claim 7 wherein the wireless system comprises an OFDM MIMO system.
 9. The method of claim 1 wherein the steps of parsing the bit stream further includes the steps of bitwise or group-wise round robin parsing to increase spatial diversity.
 10. The method of claim 9 wherein the steps of parsing the bit stream further includes the steps of bitwise round robin parsing such that one bit of the bit stream is parsed to one data stream each time.
 11. The method of claim 1 further including the steps of puncturing each spatial data stream after the step of parsing.
 12. The method of claim 11 wherein the step of puncturing for each spatial data stream is based on the channel condition.
 13. A wireless communication system, comprising: a transmitter including: a parser that parses a bit stream into multiple spatial data streams; multiple interleavers corresponding to the multiple spatial data streams, wherein each of one or more interleavers interleaves the bits in the corresponding spatial data stream by performing frequency rotation after an interleaving operation, to increase diversity of the wireless system, wherein frequency rotation is performed according to relation: r=(j−((2×i _(ss))mod 3+3×floor(i _(ss)/3))×N _(rot) ×N _(BPSC))mod N _(CBPS) , j=0, 1, . . . , N _(CBPS)−1, wherein N_(BPSC) is the number of coded bits per subcarrier, N_(CBPS) is a block size corresponding to a number of coded bits in a single Orthogonal Frequency Division Multiplexing (OFDM) symbol, j is an index after a second permutation and before a third permutation, r is an index after the third permutation, i_(SS)=0, 1, . . . , N_(SS)−1 is the index of the spatial stream on which interleaving is operating, N_(ss) is the number of spatial streams, and N_(rot) is a base rotation number in use; and a modulator that transmits the bits of each spatial data stream; and a receiver that receives and deinterleaves the transmitted bits.
 14. The system claim 13 wherein an interleaver performs a first interleaving permutation to ensure that adjacent coded bits are mapped onto nonadjacent subcarriers in one data stream for transmission, and a second permutation to ensure that coded bits are mapped alternately onto less and more significant bits of the constellation, and a third interleaving permutation varying on different spatial data streams by performing different frequency rotation to increase diversity of the wireless system.
 15. The system of claim 14, wherein the interleaver performs interleaving the bits in each spatial data stream by: (a) ensuring that adjacent coded bits are mapped onto nonadjacent sub-carriers, and (b) ensuring that coded bits are mapped alternately onto less and more significant bits of the constellation whereby long runs of low reliability (LSB) bits are avoided.
 16. The system of claim 15 wherein each spatial data stream interleaver array includes N_(row) rows and N_(column) columns of bits, wherein: ensuring that adjacent coded bits are mapped onto nonadjacent sub-carriers is according to relation: i=N _(row)×(k mod N _(column))+floor(k/N _(column)), k=0, 1, . . . , N _(CBPS)−1, ensuring that coded bits are mapped alternately onto less and more significant bits of the constellation whereby long runs of low reliability (LSB) bits are avoided is according to relation: j=s×floor(i/s)+(i+N _(CBPS)−floor(N _(column) ×i/N _(CBPS)))mod s, i=0, 1, . . . , N _(CBPS)−1, where s is determined according to s=max(N_(BPSC)/2,1), and k denotes the index of the coded bit before a first permutation.
 17. The system of claim 16 wherein the receiver includes a plurality of deinterleavers such that each deinterleaver deinterleaves the bits in a received spatial data stream.
 18. The system of claim 17 wherein each deinterleaver deinterleaves the received bits according to relations: x _(d)=(x _(dd)+((2×i _(ss))mod 3+3×floor(i _(ss)/3))×N _(rot) ×N _(BPSC))mod N _(CBPS) , x _(dd)=0, 1, . . . , N _(CBPS)−1 j _(d) =s×floor(j _(dd) /s)+(j+floor(N _(column) ×j _(dd) /N _(CBPS)))mod s j _(dd)=0, 1, . . . , N _(CBPS)−1, i _(d) =N _(column) ×i _(dd)−(N _(CBPS)−1)×floor(N _(column) ×i _(dd) /N _(CBPS)), i _(dd)=0, 1, . . . , N _(CBPS)−1, wherein the index of the coded bit before the first permutation is denoted by x_(dd), and x_(d) is the index after the first and before the second permutation, j_(d) is the index after the second permutation and before the third permutation, i_(d) is the index after the third permutation, N_(CBPS) denotes a block size corresponding to the number of coded bits in a single OFDM symbol, i_(SS)=0, 1, . . . , N_(SS)−1 is the index of the spatial stream on which this interleaver is operating, and N_(rot) is a base rotation number in use.
 19. The system of claim 13 wherein the wireless system comprises a MIMO system.
 20. The system of claim 19 wherein the wireless system comprises an OFDM MIMO system.
 21. The system of claim 13 wherein the parser parses the bit stream by bitwise round robin parsing, to increase spatial diversity.
 22. The system of claim 21 wherein the parser parses the bit stream by bitwise round robin parsing such that one bit of the bit stream is parsed to one data stream each time.
 23. A wireless apparatus, comprising: a transmitter including: a parser that parses a bit stream into multiple spatial data streams; multiple interleavers corresponding to the multiple spatial data streams, wherein each of one or more interleavers interleaves the bits in the corresponding spatial data stream by performing frequency rotation after an interleaving operation, to increase diversity of the wireless system, wherein frequency rotation is performed according to relation: r=(j−((2×i _(ss))mod 3+3×floor(i _(ss)/3))×N _(rot) ×N _(BPSC))mod N _(CBPS) , j=0, 1, . . . , N _(CBPS)−1, wherein N_(BPSC) is the number of coded bits per subcarrier, N_(CBPS) is a block size corresponding to a number of coded bits in a single Orthogonal Frequency Division Multiplexing (OFDM) symbol, j is an index after a second permutation and before a third permutation, r is an index after the third permutation, i_(SS)=0, 1, . . . , N_(SS)−1 is the index of the spatial stream on which interleaving is operating, N_(ss) is the number of spatial streams, and N_(rot) is a base rotation number in use; and a modulator that transmits the bits of each spatial data stream. 